1. Field of the Invention
The present invention relates to an air conditioner and, more particularly, to an air conditioner of heat-pump type.
2. Description of the Prior Art
A conventional air conditioner of heat-pump type encounters a problem that the operation is seriously impaired due to frosting on the outdoor heat exchanger, when the outdoor air temperature is low. In order to obviate this problem, it has been a common practice to employ a thermostatic expansion valve as the expansion device during the heating mode of operation of the air conditioner. A typical refrigeration system of a heatpump type air conditioner employing such a thermostatic expansion valve will be explained hereinunder with reference to FIG. 5.
Referring to FIG. 5, the air conditioner has a compressor 1, a four-way type reversing valve 2, an indoor heat exchanger 6, a thermostatic expansion valve 7, and an outdoor heat exchanger 5 which are connected in such a manner as to form a refrigeration system. In the heating mode operation of the air conditioner, a refrigerant flows through this system as indicated by arrows. The amount of expansion performed by the thermostatic expansion valve, i.e., the amount of restriction of the refrigerant passage in the expansion valve, is controlled in accordance with the refrigerant temperature which is sensed by a sensing bulb 11 which is secured to the wall of the refrigerant pipe between the reversing valve 2 and the input side of the compressor 1. The arrangement is such that, when frost is generated on the outdoor heat exchanger 5 during operation at low ambient air temperature, the temperature of the refrigerant into the compressor 1 is lowered correspondingly and the sensing bulb 11 which detects this temperature drop operates to restrict the area of the refrigerant passage in the expansion valve 7, in accordance with the superheating characteristics of the expansion valve 7. A capillary tube 22 connected in parallel with the expansion valve 7 has a function to reduce the extent of restriction in the expansion valve 7, thus enabling delicate control of the degree of superheating of the refrigerant, during operation of the air conditioner in the cooling mode. The capillary tube 22 also serves as a balancing capillary for maintaining system balance which may otherwise be lost due to closing of the expansion valve 7 during operation in the cooling mode or during suspension of heating mode operation, when the ambient air temperature is not so high.
A bypass pipe 3 having a two-way valve 4 is connected between the input side of the compressor 1 and the output side of the outdoor heat exchanger 5.
The thermostatic expansion valve has a superheating characteristics as shown in FIG. 6. It will be seen that the superheating degree .DELTA.t.sub.1 obtained when the temperature sensed by the sensing bulb is comparatively low as represented by T.sub.1 is smaller than the superheating degree .DELTA.t.sub.2 obtained when the temperature sensed by the sensing bulb is comparatively high as represented by T.sub.2. The superheating degree, therefore, has to be determined such that the compressor motor winding temperature does not exceed an allowable temperature even in overloaded operation in the heating mode. This inevitably requires that the superheating degree is set at a comparatively low level. FIG. 6 is a diagram showing saturation curve which represents the relation between pressure and temperature of a refrigerant. More specifically, axis of ordinate represents the pressure, while the axis of abscissa represents the temperature. A solid-line curve (1) shows the saturation cuve of a refrigerant such as R.sub.22, while a solid-line curve (2) represents the change in the temperature of the refrigerant, i.e., change in superheating degree, across the thermostatic expansion valve 7.
When the air conditioner having a refrigeration system with such a small degree of superheating in the expansion valve operates in the heating mode at a comparatively low ambient air temperature, a phenomenon known as "liquid back" tends to occur with the result that the temperature of the compressor is lowered abnormally. On the other hand, the frost on the outdoor heat exchanger grows so that the air-flow resistance across the outdoor heat exchanger is progressively increased as shown by curves a, b and c in FIG. 7A. This in turn causes a reduction in the speed of the fan of the blower provided on the outdoor heat exchanger, as shown in FIG. 7B, causing a further reduction in the air flow rate through the outdoor heat exchanger. This inconveniently lowers the evaporation temperature in the outdoor heat exchanger so as to promote the frosting and to further lower the temperature of the compressor. FIGS. 7A and 7B show air flow rate characteristics of the blower on the outdoor heat exchanger. In FIG. 7A, the axis of ordinate represents the difference in head or pressure between the suction side and the discharge side of the blower, while the axis of abscissa represents the air flow rate. In FIG. 7B, the axis of ordinate and the axis of abscissa represent, respectively, the speed of the fan of the blower and the air flow rate.
The air conditioners of the kind described above usually employ a defrosting system which relies upon recirculation of the hot refrigerant from the compressor through the outdoor heat exchanger. In such a defrosting system, the defrosting heat quantity is given as the sum of the heat accumulated in the compressor and the compressor power requirement. Thus, the quantity of heat accumulated in the compressor shares a significant portion of the defrosting heat quantity. In case of a heavy frosting on the outdoor heat exchanger, however, the heat accumulated in the compressor is soon consumed away so that an impractically long defrosting time is required for melting the heavy frost. Needless to say, the heavy frosting causes a serious reduction in the heating power of the air conditioner, before the defrosting is commenced.
Thus, the conventional air conditioners have suffered a problem in that the mean heating capacity tends to be reduced when the ambient air temperature is low, thus failing to meet the demand for comfort. Air conditioners of the type described are shown, for example, in Japanese Utility Model Laid-Open Nos. 61873/1981 and 67969/1981.